A sound wave of 40.0 cm wavelength enters
the tube shown in the figure at the source end. What must be the smallest
radius r of the curved path such that a maximum intensity will be heard at
the detector end?

The smallest value of r is
cm

Answer

But

If we take the smallest
non-zero DL
in terms of the wavelength we get

Sound travels through two
different paths. The path length difference between the two sound
waves is

Since we want a
maximum sound at the detector, the relationship between the path length
difference and the wavelength is